Nickel metal-hydride (NiMH) batteries, and methods for their manufacture, are well known. Such batteries produce electrical current by the electrochemical reaction between NiOOH and a metal hydride (MH) in an aqueous alkali metal hydroxide (e.g. KOH) electrolyte according to the following reactions:Cathode (+): NiOOH+H2O+eNi(OH)2+OH—  (1)Anode (−): MH+OH—M+H2O+e  (2)Overall: MH+NiOOHM+Ni(OH)2  (3)where “M” is a metal (i.e. alloyed or not) capable of electrochemically storing and releasing hydrogen in a KOH environment.
Suitable metal hydrides for these batteries are primarily formed from transition metals (i.e. the Group IIIA–VIIIA metals of the Periodic Table including the lanthanide series [i.e. rare earth] metals). Of these, (1) the AB2-based phase where A=Zr, Ti, or Mg, and B=Ni, V, Cr or Mn, such as ZrMn2, ZrCr2, (2) the AB5-based phase where A=rare earths such as La or mischmetal, and B=transition metals such as Ni, have been found to be particularly effective, and include, inter alia, such alloys as LaNi5, CaNi5, ZrV2, and alloys thereof with such other metals as aluminum, tin, manganese, cobalt, silicon, chromium, calcium, magnesium, lithium, carbon, titanium vanadium, iron, yttrium, nickel, copper, zirconium, niobium, molybdenum lanthanum, tungsten and rhenium, and various rare earth metals. Moreover, Ti3Ni2 and Cr-doped or Al-doped Ti3Ni2 have shown to be effective hydride formers. The MH electrode may be prepared by mixing ball-milled MH powder with carbon powder and a binder (e.g. polyethylene), and hot pressing it onto a suitable support/current collector. Preferably, electrocatalytic metals (e.g. AB3-type materials such as MoCo3, MoNi3, WNi3, ZrPt3, inter alia) will be homogeneously distributed throughout the MH-C mix to provide good hydrogen storage levels and electrocatalytic activity. Hot KOH etching of the electrodes enhances their activation. A more detailed discussion of MH electrodes can be found in F. Feng, M. Geng, D. Northwood, Electrochemical behavior of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review, International Journal of Hydrogen Energy 26, 725–734, (2001), which is incorporated herein by reference.
NiOOH electrodes are made by (1) first preparing a paste comprising Ni(OH)2 and a binder [e.g. polytetrafluoroethylene (PTFE), polyethylene (PE), and/or polyvinyl chloride (PVC)] in a suitable solvent [e.g. a mixture of tetrahydrofuran (THF) and dimethylformamide (DMF), or a mixture of t-butanol and water], (2) spreading the paste onto a suitable support/current collector (e.g. Ni screen, mat, expanded metal or foam), and (3) heating it (e.g. 130° C.–250° C.) for about 5–10 minutes to drive off the solvent, and bind the paste's components together and to the support/current collector. Carbon particles may be added to the mix to enhance its conductivity. The Ni(OH)2 converts to NiOOH when the battery is charged.
Low temperature, alkaline H2—O2/air fuel cells (hereafter AFC) are well known in the art and include cells that use either pure oxygen or air as the oxidant (hereafter O2/air or oxygen/air). Such fuel cells produce electrical current by the electrochemical reaction between H2 and O2/air at temperatures preferably below about 80° C. in an aqueous KOH electrolyte according to the following reactions:anode: H2+2OH—→2H2O+2e-  (4)cathode: O2+2H2O+4e-→4OH—  (5)overall: 2H2+O2→2H2O  (6)
Electrodes for such AFCs are known as “gas-diffusion” electrodes, which (1) comprise a gas diffusion layer and a reaction layer, (2) are inert to the reactants and the electrolyte, and (3) are electronically conductive, and porous—having a pore system through which gas can be readily transferred, but which resists leakage of electrolyte into the gas chambers or flood the electrode. The use of small pores in the range of 1–100 microns provide strong capillary forces that resist free flow of electrolyte through the electrode. Dual porosity electrodes having larger pores on the gas side, and smaller pores on the electrolyte side, have proven to be quite effective. Gas-diffusion electrodes have been made from self-supporting porous carbon or sintered metal (e.g. nickel), or by securing a porous active material (e.g. carbon) to a porous current collector. Plastic-bonded electrodes (i.e. catalyst particles in a plastic matrix) have also been used successfully. A catalyst (e.g. Pt, Pt—Pd, Ni, Ag, NiO etc.) suitable to effecting the anode reaction or the cathode reaction, as appropriate, is contained within the porous active material. Wetproofed gas-diffusion electrodes have (1) a hydrophobic side that confronts the reactant gas (i.e. H2 or O2/air) and is not wettable by the electrolyte, and (2) an opposing, hydrophilic side that confronts the electrolyte and is wettable thereby. A catalyzed reaction zone exists within the electrode, between the two sides, where the three phases (i.e. liquid electrolyte, gaseous reactant, and solid current collector/catalyst) meet to form an anode or cathode reaction site, as appropriate. Oft times, the electrolyte-confronting side of the electrode is covered with an electrolyte-absorbent material, e.g. a material like that used as a separator in NiMH batteries. Since Wm. Grove's invention of the fuel cell, and F. T. Bacon's adaptation thereof to aqueous KOH electrolytes, the techniques and materials for manufacturing gas diffusion electrodes have advanced significantly, have become highly developed in the art, and are all useful in the practice of the present invention.
Early AFCs used noble metal catalysts (e.g. Pt) for both the anode and cathode. Non-noble, metal hydride anode catalysts have been proposed that both (1) catalyze the formation of atomic hydrogen and water, and (2) store hydrogen within the anode for release when needed. Such electrodes are known as H2-storing electrodes and contain non-noble metal catalysts that comprise rare-earth/mischmetal alloys, zirconium and/or titanium or mixtures thereof, and include, for example, MaCobMncFedSne (where “M”=0.1–60 atomic % Ti, 0.1–40 atomic % Zr, 0–60 atomic % V, 0.1-atomic % Ni, and 0–56 atomic % Cr), “b”=0–7.5 atomic %, “c”=13–17 atomic %, “d”=0–3.5 atomic percent, “e”=0–1.5 atomic %, and a+b+c+d+e=100%. Catalytic regions will preferably be distributed throughout the anode material. Such catalysts are discussed in more detail in PCT Patent Application US01/07864 which was published as International Publication Number WO 01/69701 on 20 Sep. 2001, and is herein incorporated by reference. H2-storing AFC anodes comprising Raney nickel and H2-absorbing alloys have been proposed, and are discussed in EPO Patent 277332 (i.e. EP Patent Application no. EP 87118803 filed Dec. 18, 1987) which is also incorporated herein by reference.
Similarly, AFC cathodes capable of storing and releasing oxygen have been proposed. Such electrodes are known as O2-storing electrodes and comprise a mixture of a non-noble catalyst (e.g. carbon) and an active material capable of reversibly storing energy through a redox (reduction/oxidation) couple mechanism such as provided by metal/oxide couples selected from the group consisting of copper/copper oxide, silver/silver oxide, zinc/zinc oxide, cobalt/cobalt oxide, and cadmium/cadmium oxide, inter alia. The NiOH/NiOOH redox couple is also considered to be useful. AFC cathodes made from such redox couples are discussed in more detail in U.S. patent application Ser. No. 09/797,332 published Oct. 25, 2001 as U.S. 2001/0033959 A1, which is incorporated herein by reference. AFCs containing H2-storing anodes and O2-storing cathodes discussed above are said to also be capable of functioning as electrolyzers for electrolytically dissociating water into hydrogen and oxygen which is stored in the anode and cathode respectively (e.g. during regenerative braking of vehicles powered by such AFCs).
The electrolytes used for both the NiMH batteries and AFCs are essentially the same, and comprise about 20% to about 50% aqueous alkali metal hydroxide, preferably about 30% KOH for its superior conductivity. The electrolyte may either be flowed between the electrodes, or be maintained static therebetween, as in a quiescent pool, or by absorption into a porous matrix (e.g. asbestos cloth or mat, or a cast polymer film), a.k.a. “separator”, that engages, and separates, adjacent electrodes one from the next. The electrolyte-absorbing matrix/separator may be made with finer pores than the adjacent electrodes which is effective in preventing flooding of the electrodes. A popular matrix/separator material is ZIRFON® which comprises a cast film (i.e. about —200 μm to about 350 μm thick) comprising zirconium oxide and polysulfone. ZIRFON® is discussed in Ph. Vermeiren, W. Adriansens and R. Leysen, ZIRFON® A new Separator for Ni—H2 Batteries and Alkaline Fuel Cells, Int. J. Hydrogen Energy Vol. 21, No. 8, pp. 679–684, 1996, which is incorporated herein by reference. Circulating electrolytes are preferred for AFCs as a means to carry away the reaction water, to help manage the AFC's temperature, and to permit external purification of the electrolyte (i.e. remove carbonates formed therein by reaction with CO2 in the air) before it is recirculated back to the fuel cell.